California Code, Public Resources Code - PRC § 49178

(a) An issue of bonds is hereby defined to be the aggregate principal amount of all of the bonds authorized to be issued in accordance with a proposal submitted to and approved by the electors of the district, but no indebtedness is deemed to have been contracted until bonds have been sold and delivered and then only to the extent of the principal amount of the bonds so sold and delivered.

(b) The district board issuing bonds may, in its discretion, divide the aggregate principal amount of the issue into two or more divisions or series and fix different dates for the bonds of each separate division or series. If any authorized issue is divided into two or more divisions or series, the bonds of each division or series may be made payable at the time or times fixed by the district board, separate and distinct from the time or times for the payment of bonds of any other division or series of the same issue.